An organic electroluminescent display panel (hereinafter referred to as organic EL display panel) may be constructed to contain an image display array comprised of a plurality of light-emitting pixels arranged in intersecting rows and columns on a light-transmissive substrate. It will be understood that the term "display panel" includes a construction which contains only a single light-emitting pixel or a single light-emitting region shaped as a symbol or icon.
In one form, an organic EL display panel is constructed as follows: on a light-transmissive substrate are provided a series of parallel laterally spaced light-transmissive anode electrodes. An organic EL-medium is then formed on the light-transmissive substrate and over the light-transmissive anode electrodes. The EL-medium typically comprises several overlying thin layers of organic materials which, in combination, are capable of emitting light. A plurality of parallel laterally spaced cathode electrodes is disposed over the EL-medium in an oriented direction with respect to the anode electrodes. In order to provide a permanent and reliable electrical contact to each of the anode and cathode electrodes on one hand, and to provide a reliable electrical connection between each of the electrodes and an electrical drive signal generator on the other hand, a metallized leader is formed so as to be in electrical contact with one termination of each of the electrodes. Each metallized leader, in turn, connects a corresponding anode electrode or a corresponding cathode electrode via an electrical connector (for example, a wire connector) to an electrical drive signal addressing system.
In another form, an organic EL display panel is constructed on a light-transmissive substrate as follows: at least one thin-film transistor (TFT) is formed on the substrate by semiconductor processing techniques known to those skilled in the art. An organic EL light-emitting pixel is formed to be associated with the TFT by providing a light-transmissive anode electrode which is electrically connected to a drain contact of the TFT to provide a positive electrical signal to the anode electrode when both, a source input signal and a gate input signal provided to the TFT from external signal generators, cause the TFT to be in an "on-state." An organic EL-medium is disposed over the anode electrode, and a cathode electrode is provided over the EL-medium. The cathode electrode requires a metallized leader which is environmentally stable and serves to provide a reliable electrical connection between the cathode electrode and an electrical connector which connects the electrode to a ground bus or to a suitably chosen bias voltage source. In contrast to the cathode electrode, the source, drain, and gate electrodes of the TFT, as well as their respective source leads, drain leads, and gate leads and associated bond pads, generally do not require additional metallized leaders because these elements are fabricated from environmentally stable and abrasion-resistant materials, for example, from polysilicon, aluminum-silicon alloys, and tungsten silicide materials.
When an electrical drive signal is applied between any one of the anode electrodes and any one of the cathode electrodes (or when the TFT is in an on-state) such that a drive signal is more positive at the anode electrode than the cathode electrode, electrons injected into the EL-medium from the cathode electrode and holes injected into the EL-medium from the anode electrode, recombine in the EL-medium and cause light emission therefrom. In the aforementioned two constructions of the organic EL display panel, the light is emitted through the light-transmissive anode electrode and through the light-transmissive substrate for viewing by an observer.
The metallized leaders are formed of a metal or of a metal alloy layer of sufficient thickness and width to provide a desired electrical conductivity and to have mechanical integrity and abrasion resistance desirable for the attachment of the electrical connectors to the metallized leaders. Thus, the metallized leaders are optically opaque.
The anode electrodes are preferably formed from a light-transmissive indium-tin oxide (ITO), and the cathode electrodes are preferably formed by vapor deposition of a metal alloy material, for example, a magnesium silver alloy material. The cathode electrodes are usually optically opaque.
Both the EL-medium and the cathode electrodes are subject to degradation caused by moisture and/or oxygen when a display panel is operated under ambient environmental conditions. Such degradation is accelerated at elevated temperature.
Accordingly, the EL-medium and the cathode electrodes need to be protected. Solvent-coated protective layers, such as solvent-coated organic resins, can not be used to seal the entire surface of the display panel because the organic EL-medium is adversely or catastrophically affected by solvents.
A protective cover sealed to an organic EL display panel can offer significant environmental protection of an active region of the panel if an effective seal can be provided between the cover and the display panel along a perimeter seal band which extends on the substrate outside the active area of the panel having the plurality of light-emitting pixels and intersecting a portion of the metallized leaders associated with the anode and cathode electrodes (or the source, drain, and gate lines, as the construction of the display panel may require) so that terminal portions of the leaders remain accessible as bond pads for bonding electrical connectors thereto. Solvent-free heat curable resins, and hot-melt adhesives have been commercially available and have been used to seal a protective cover on an organic EL display panel.
To provide an effective seal, heat-cured resin seals can require curing conditions at substantially elevated temperature (90-150.degree. C.) for an extended period of time (20-60 minutes). Hot-melt adhesives are typically "melted" at a temperature of about 150.degree. C. to form a bead of liquid adhesive on a surface of the cover. The display panel is oriented with respect to the cover and is pressed against the surface of the cover to form a perimeter seal between the organic EL panel and the cover.
Cover seals formed from the above-mentioned perimeter seal-forming materials have three principal disadvantages: (1) measurable degradation of the light intensity of the light emitted by the pixel of the organic EL display panel can occur due to partial degradation of the organic EL-medium caused by the elevated temperature and extended curing time requirements; (2) elevated temperature curing requirements can result in long-term degradation of the perimeter seal most likely attributable to stress forces evolving in the perimeter seal due to a mismatch of thermal expansion coefficients between the seal forming material and the cover and/or the display panel; and (3) as a consequence of elevated temperature and extended curing time requirements, the practical throughput of sealed display panels in a manufacturing environment can be limited.
The aforementioned problems associated with heat-cured resin seals and with hot-melt adhesive seals can, in principle, be overcome by using a class of commercial seal-forming materials variously known as radiation-curable resins or as ultraviolet (UV)-curable adhesives to seal a protective cover over an organic EL display panel by a perimeter seal. However, one substantial constraint in forming a perimeter seal of radiation-curable resin is that of potentially insufficient or incomplete cure of the seal in the regions immediately above the optically opaque metallized leaders due to optical shadowing caused by the leaders.
Highly reactive radiation-curable resins, such as acrylic resins, can cure "laterally" under relatively wide (1-2 mm) metallized leaders. Unfortunately, cured acrylic resin perimeter seals have poor moisture resistance, and do not maintain an effective moisture seal under required high humidity and elevated temperature stress testing of organic EL display panels.
Less reactive radiation-curable resins such as, for example, epoxy-based resins, offer acceptable moisture resistance of a fully cured perimeter seal under stress testing conditions of an EL display panel. However, these less reactive resins exhibit poor or incomplete curing of a perimeter seal in shadowed regions created by metallized leaders of a width dimension as used with the acrylic resins described above.
In order to retain the desirable moisture resistance feature of a perimeter seal formed of a radiation-cured epoxy-based resin, and to achieve complete curing of the seal in the shadowed regions of the metallized leaders, it is tempting to contemplate a reduction of the width or size of metallized leaders to a dimension at which even this less reactive resin can be cured "laterally" in the shadowed regions. However, such contemplated width or size reduction must be balanced against the requirement that the metallized leaders must remain sufficiently electrically conductive to conduct electrical current of a magnitude which will ensure optimum performance of an organic EL display panel.